Business Combinations, Goodwill by Segment (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
BUSINESS COMBINATIONS, GOODWILL AND OTHER INTANGIBLE ASSETS [Abstract]
Finite-Lived Intangible Assets, Net	$ 284.0
Identifiable intangibles with indefinite useful lives	8.8
Goodwill [Line Items]
Servicing Asset at Amortized Cost, Other than Temporary Impairments	6.5	$ 4.3
Goodwill [Roll Forward]
Goodwill	2,709.3	2,579.3
Goodwill Additions, net of adjustments	8.9	29.2
Goodwill, Translation Adjustments	(35.3)	60.4
Goodwill	2,682.9	2,668.9
Investment Management [Member]
Goodwill [Roll Forward]
Goodwill	16.5	15.4
Goodwill Additions, net of adjustments	0.0	0.0
Goodwill, Translation Adjustments	(0.3)	0.7
Goodwill	16.2	16.1
Americas [Member] \| Reportable Subsegments [Member]
Goodwill [Roll Forward]
Goodwill	1,412.2	1,406.1
Goodwill Additions, net of adjustments	6.9	4.5
Goodwill, Translation Adjustments	(0.8)	0.4
Goodwill	1,418.3	1,411.0
EMEA [Member] \| Reportable Subsegments [Member]
Goodwill [Roll Forward]
Goodwill	957.6	851.7
Goodwill Additions, net of adjustments	0.1	18.2
Goodwill, Translation Adjustments	(27.7)	52.1
Goodwill	930.0	922.0
Asia Pacific [Member] \| Reportable Subsegments [Member]
Goodwill [Roll Forward]
Goodwill	323.0	306.1
Goodwill Additions, net of adjustments	1.9	6.5
Goodwill, Translation Adjustments	(6.5)	7.2
Goodwill	318.4	$ 319.8
Other Intangible Assets [Member]
BUSINESS COMBINATIONS, GOODWILL AND OTHER INTANGIBLE ASSETS [Abstract]
Finite-Lived Intangible Assets, Net	95.5
Mortgage servicing rights [Member]
BUSINESS COMBINATIONS, GOODWILL AND OTHER INTANGIBLE ASSETS [Abstract]
Finite-Lived Intangible Assets, Net	$ 188.5

-- None. No documentation exists for this element. --

No definition available.

Name:	jll_BusinessCombinationsGoodwillAndOtherIntangibleAssetsAbstract
Namespace Prefix:	jll_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The aggregate amount of goodwill acquired in the period and allocated to the reportable segment. The value is stated at fair value based on the purchase price allocation. Also includes purchase price allocation adjustment for allocations not yet complete at the prior year end.

+ References

No definition available.

+ Details

Name:	jll_GoodwillAdditionsNetOfAdjustments
Namespace Prefix:	jll_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

+ Details

Name:	us-gaap_Goodwill
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of foreign currency translation gain (loss) which increases (decreases) an asset representing future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

+ Details

Name:	us-gaap_GoodwillForeignCurrencyTranslationGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_GoodwillLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

Name:	us-gaap_GoodwillRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

+ Details

Name:	us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of other-than-temporary impairment (OTTI) applied to contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 4
-Subparagraph (a)(6)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122739-111746

+ Details

Name:	us-gaap_ServicingAssetAtAmortizedCostOtherThanTemporaryImpairments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=jll_InvestmentManagementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=us-gaap_AmericasMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=us-gaap_ReportableSubsegmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=us-gaap_EMEAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=us-gaap_AsiaPacificMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_OtherIntangibleAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=jll_MortgageservicingrightsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: